Heat pumps: an introduction

Heat Pumps take heat from the ground, air or water and use it for
space heating, and to heat hot water. They are like a fridge
in reverse. The fridge takes heat from the food you put in it, and
pumps that into the kitchen, keeping the food cold. The heat pumps take
heat from the ground (air or water) and pumps it into your house, keeping it
warm.

Is a heat pump suitable for my home?

Heat pumps arenâ€™t suitable for every home. They work best in well insulated buildings, and can be a good choice in new build. If you are hoping that a heat pump will lower your heating costs, this is unlikely if you are replacing a mains gas boiler. If you currently heat with oil, LPG or electricity, you may benefit financially.

To install a ground source heat pump you need plenty of outside space for the pipework which is generally buried in trenches. It can also be inserted in a borehole, which costs a bit more. In this case, you will need suitable access for the drilling machinery. Air source heat pumps take up much less space, but you do need a bit of distance between it and your neighbours.

Heat pumps heat water to a
lower temperature than traditional boilers. As a result the ideal place for them is an extremely well insulated house with underfloor heating. You can use a heat pump with radiators, but to get the
same level of heat you will need larger radiators. Many older buildings are not energy efficient enough to use
underfloor heating or low temperature radiators.

With a traditional boiler the hot water cylinder tends to be heated to 60C or higher. With a heat pump, the hotter you heat your water the more electricity you use, which leads to higher running costs. 40 - 50C is generally hot enough for washing up and bathing, but the temperature in the cylinder needs to be boosted to over 60C once a week to avoid the danger of legionella. Some heat pumps come with an integrated immersion heater.

Because they do not take up much space, air source heat pumps are more likely to be used in flats and in urban areas, particularly in places where there is no mains gas supply, or to replace electric heating. Noise from the fan must be below 42 decibels from a metre away to meet permitted development rules.

For a ground source heat pump you need space outside to dig trenches, or
sink a borehole, for the ground loop. A typical installation ranges from 6 to
12 kW in size. Youâ€™ll need trenches that are 1.5 to 2 metres deep and long
enough to lay 50 to 80m of pipe per kW or 10m of slinky (coiled) pipe. As a rule of thumb, you'll need twice the area of the property for the ground arrays.

Boreholes use less land, but are more expensive to drill. They tend to need
between 20 and 50m of pipe per kW. Boreholes are usually 100 â€“ 150mm in
diameter and up to 120m deep. More than one pipe can be put in each borehole,
but some systems will need more than one borehole.

The geology of the ground around your property is important in determining whether a ground source heat pump is suitable. For example, sandy soil drains fast and does not hold heat well. Heat pumps will not perform well in an area with this soil.

2. They are sensitive to design and commissioning. However, between the phase one trial in 2010 and this trial the EST says that the reasons for previous underperformance are understood, and have been addressed by the new, improved MCS standards.

3. While most people taking part in the trial were happy with the heating and hot water provided by their system, they wanted more information. Installers and manufacturers should work to develop customer understanding of their system.

4. Different aspects of a heat pump system have different impact on its performance. In particular, auxilliary and immersion heaters can have a significant impact on the efficiency of the system, and customers will benefit from knowing that.

5. It's often said that the most efficient way to run a heat pump system is to have it running continuously. However, the trial found that a number of well-performing systems were controlled non-continuously, and their owners were satisfied with their performance.

6. Users' understanding of their systems is varied. The data suggests that if people understand about system design and control, the overall performance is likely to be better.

Seasonal Performance Factor (SPF)

The performance of heat pumps is rated as a seasonal performance factor (SPF). It is the total useful heat generated from the heat pump in a year divided by the annual electricity consumption. For example an SPF of 3
indicates that the system will give an average three units of heat energy for each one unit of electricity used.

To be considered 'renewable' (under EU legislation) heat pumps must have a SPF of at least 2.5, and this is the minimum performance that is eligible for the domestic renewable heat incentive.

The domestic renewable heat incentive will pay on renewable heat only, so the more efficient the heat pump, the greater the payment you will receive. This makes it worth investing a bit more in your system to make sure you have a good quality heat pump, with suitably sized radiators or underfloor heating, installed by a reputable installer (MCS accredited, and recommended by the manufacturer). Click the link to see a table illustrating the difference in RHI payments for systems with different star ratings.

However, there is likely to be a balance to be struck. The upfront cost of getting the most efficient system - ie installing new radiators or underfloor heating - may be prohibitive. In this case a cheaper, slightly less efficient system with less disruption and a lower purchase cost, but higher running costs may be a better alternative.

Choosing a good heat pump installer

1. Make sure that responsibility and liability for the complete
installation lies with one company, ideally with a contract to guarantee
consistency in after-sales service. The EST research found that
often there was no single contractor responsible, and installations
might involve a ground works contractor, a plumber, a heat pump
installer, and an electrician, none of whom had liability for the
overall system.

2. Ask the installer to explain how they have calculated the
appropriate type and size of heat pump for your house, and explain how
they calculated the heat demand of your house. The calculation should take account of your property size and location, how energy efficient it is, and whether the heat pump is for space heating, water heating or both.

3. Check how experienced the installer is and whether previous customers are happy. If possible, talk to people who have had their heat pump at least a year.

4. Make sure both installer and the product they are installing are
MCS accredited, so you will be eligible for the Renewable Heat
Incentive. Also check that they are members of the Renewable Energy Consumer Code (RECC).

These prices do not include the cost of installing an underfloor heating system which would be around Â£2,000 depending on the size of your house or larger radiators suitable for lower flow temperatures.

Renewable heat incentive

The domestic Renewable Heat Incentive

The domestic renewable heat incentive is a boiler replacement scheme, aiming to bridge the gap between the cost of a replacement oil or LPG boiler and the cost of a renewable heating system. Air, ground and water source heat pumps are all eligible systems.

RHI rates are:Air to water heat pumps: 7.3p per kWhGround or water source heat pump: 18.8p per kWh Payments are index-linked and paid quarterly in arrears for seven years.

To apply you will must have a green deal assessment (unless you are a self builder, in which case an EPC is required), and install loft and cavity wall insulation if it gets a green tick on the green deal advice report.

The RHI will be paid on each kWh of renewable heat generated by your biomass boiler or stove.

To calculate the payments you will receive, follow the formula in this example:

House with a heat demand (space heating plus hot water) of 18,000kWh per year. The heat demand figure will be taken from your EPC.

Installing an air source heat pump with an efficiency rate averaged over the whole year (seasonal performance factor or SPF) of 3 - ie it generates an average of 3kWh of heat for every 1kWh of electricity used.

Tariff rate of 7.3p per kWh.

The RHI is only paid on the renewable element of the heat, not the electricity used. To find that figure use the formula 1 - 1/SPF:

Application is via the Ofgem website, where there is also a lot of helpful information and advice. You can also contact Ofgem via Twitter @AskDomesticRHI.

Heat pumps are approved measures under the Green Deal, so green deal finance may be available to help with the upfront cost of installing a domestic heat pump. This is repaid through the savings in your electricity bills.

Non-domestic Renewable Heat Incentive (RHI)

Rates are:

4.9p per kWh for small heat pumps, less than 100kWth3.6p per kWh for large heat pumps of 100kWth+

Ground source and water source heat pumps heating water are eligible for the non-domestic RHI. Air source heat pumps may be added to the scheme after further research into cost and performance. Results of the consultation are expected in autumn 2013. Click here for frequently asked questions on phase 1 of the RHI.

How do heat pumps work?

extract heat from the air using an evaporator coil. This looks like the big fans on air conditioner units and is fixed on an outside wall of the building.

Using mains electricity, the heat pump boosts the heat from the air to the level needed by the heating system, heating water in a buffer tank.

The heating system is fed from the buffer tank.

Air to air heat pumps extract the heat from outside, boost it to the required level, and blow out hot air into a room, or through vents around the building. They can also be used for cooling (air conditioning). Air to air heat pumps are not eligible for the renewable heat incentive.

Ground source heat pump

A long loop of pipe, filled with water and anti-freeze, is buried
in the earth. Depending on available space it can be in a trench at
least 1.5m deep or down a borehole

The liquid in the pipe (or ground loop) absorbs heat from the ground which is a fairly stable 8 - 12 degrees C all year round

As it passes through an electrically powered heat pump, the
absorbed heat is extracted, and the liquid goes back into the
underground loop

Using mains electricity, the heat pump boosts the heat from the
ground to the level needed by the heating system, heating water in a
buffer tank

Planning permission for heat pumps

Ground and water source heat pumps are permitted developments.

Air source heat pumps are also permitted development as long as they meet a long list of additional criteria. Noise is one of the main issues. To meet the MCS020 planning standards, noise from ASHPs must be below 42dB at a position one metre external to the centre point of any door or window to a habitable room of a neighbouring property as measured perpendicular to the plane of the door or window.

It is a condition of permitted development that the ASHP can only be used for heating purposes.